The sheathing of metallic reinforcers with thermoplastic materials, such as for example a polyamide or polyester, has been known for a very long time, especially so as to protect these reinforcers from various types of external attack such as oxidation or abrasion, or else for the purpose of structurally stiffening, by bonding them together, various groups of threads or thread assemblies such as cords, and thus increasing particularly their buckling resistance.
Such composite reinforcers, together with their use in rubber articles such as pneumatic tires, have been described in many patent documents.
Patent application EP 0 962 576 has for example described a reinforcer, made of steel or an aramid textile, sheathed by a thermoplastic material such as a polyester or polyamide, for the purpose of improving its abrasion resistance.
Patent application FR 2 601 293 has described the sheathing of a metal cord with a polyamide so as to use it as a bead wire in a pneumatic tire bead, this sheathing advantageously enabling the shape of this bead wire to adapt to the structure and to the operating conditions of the bead of the tire that it reinforces.
Patent documents FR 2 576 247 and U.S. Pat. No. 4,754,794 have also described metal cords or threads that are doubly-sheathed or even triply-sheathed by two or even three different thermoplastic materials (e.g. polyamides) having different melting points, with the purpose, on the one hand, of controlling the distance between these threads or cords and, on the other hand, of eliminating the risk of wear by rubbing or of corrosion, in order to use them as a bead wire in a pneumatic tire bead.
These reinforcers thus sheathed with a polyester or polyamide material have, apart from the aforementioned advantages of corrosion resistance, abrasion resistance and structural rigidity, the not insignificant advantage of them being able to be subsequently bonded to diene rubber matrices using simple textile adhesives called RFL (resorcinol-formaldehyde-latex) adhesives comprising at least one diene elastomer, such as natural rubber, which adhesives are known to provide satisfactory adhesion between textile fibres, such as polyester or polyamide fibres, and a diene rubber.
Thus, it may be advantageous to use metal reinforcers not coated with adhesive metal layers, such as with brass, and also surrounding rubber matrices containing no metal salts, such as cobalt salts, which are necessary as is known for maintaining the adhesive properties over the course of time but which significantly increase, on the one hand, the cost of the rubber matrices themselves and, on the other hand, their oxidation and ageing sensitivity (see for example the patent application WO 2005/113666).
However, the above RFL adhesives are not without drawbacks: in particular they contain as base substance formaldehyde, a substance which it is desirable long-term to eliminate from adhesive compositions because of the recent changes in European regulations regarding this type of product.